Method of forming a coating

ABSTRACT

A method comprises the following steps:
         a) Providing a body having an internal surface which defines an internal pathway within the body, the body having an inlet and an outlet both communicating with the internal pathway;   b) Introducing a liquid solution into the internal pathway so as to fill at least a portion of the internal pathway with the liquid solution, the liquid solution comprising a solute capable of undergoing thermal decomposition;   c) Heating the liquid solution while the liquid solution fills said at least a portion of the internal pathway to a sufficient temperature so that the solute undergoes thermal decomposition to form a decomposition product within said at least a portion of the internal pathway.       

     The heating step forms a coating comprising the decomposition product on at least a part of the internal surface that borders the internal pathway.

The invention relates to a method of forming a coating within aninternal pathway, or within internal spaces of a porous body.

There are many known applications where it is desirable to form acoating within an internal pathway. In one such application, a poroussolid coating is formed on an internal surface of a tube so as to make acolumn that may be used for chromatography. In another such application,a catalytic coating is formed on an internal surface of a channel sothat chemical reactions may be performed within the channel. In yetanother such application, a catalytic coating may be formed on aninternal surface of a porous body so as to allow chemical reactions tobe performed within the porous body.

A known type of method for forming a coating within an internal pathwayinvolves deposition of a coating material from a sol containing solidparticles of the coating material suspended in a liquid. The liquid isevaporated leaving the solid particles which form the coating. Examplesof this are described in an article by Cherkasov, Ibhadon and Rebrov,entitled: Novel synthesis of thick wall coatings of titania supported Bipoisoned Pd catalysts and application in selective hydrogenation ofacetylene alcohols in capillary microreactors, Lab Chip, 2015, 15, 1952.In one example in this article, a titanium dioxide sol was introducedinto a fused silica capillary tube. The capillary tube was heated tocause evaporation of the liquid leaving a porous coating of titaniumdioxide. In another example, Bi-poisoned palladium catalyticnanoparticles were mixed with a titanium dioxide sol and the mixture wasintroduced into a fused silica capillary tube. The capillary tube washeated to cause evaporation of the liquid leaving a coating comprising aporous solid titanium dioxide and the Bi-poisoned palladiumnanoparticles, with the nanoparticles being supported within the poroussolid titanium dioxide.

Despite significant advances taught by the article of Cherkasov, Ibhadonand Rebrov, the formation of coatings by deposition from sols suffersfrom drawbacks. In particular, sols of suitable coating materials, suchas metal oxides, are viscous, non-Newtonian fluids that tend to clognarrow capillary tubes. Liquid evaporation tends to be slow requiringlong preparation times. It is often difficult to prepare thick coatings(greater than a few μm) using a single deposition step. It can also bedifficult to prepare coatings on long tubes (greater than about 1 m). Inaddition sols tend to be unstable over time and so use of freshly madesols is often required.

US 2004/0033319 A1 ('319) discloses a method of forming a coating on theinner surface of a tube. The method comprises providing a liquidcontaining an organic metal compound within the tube. Heat is appliedand the organic metal compound decomposes to form the coating. However,unlike the current invention, in which at least a portion of the lengthof an internal passageway is filled with a liquid solution, '319 teachesthat the liquid should be provided in the tube as a thin film coating onthe inner surface of the tube and that it is the thin film coating thatis subject to decomposition. The approach adopted in '319 can lead to anuneven coating because the thin film of liquid can become unevenlydistributed on the inner surface of the tube before decomposition iscomplete. The '319 document recognizes this problem and attempts toovercome it by partially decomposing the liquid film with either UVirradiation or ozone treatment to increase viscosity of the liquid filmbefore decomposition is complete. Nevertheless, it is still desirable toprovide improved coating methods.

U.S. 2015/0147562 A1 ('562) discloses a method in which channels andpores of a porous body are coated with a layer of phosphorous-containingalumina. The '562 document also relies on forming the coating from athin film coating of liquid.

In accordance with a first aspect of the invention, there is provided amethod of forming a coating within an internal pathway, comprising:providing a body having an internal surface which defines an internalpathway within the body, the body having an inlet and an outlet bothcommunicating with the internal pathway for passage of a fluidsuccessively into the inlet then through the internal pathway and thenout of the outlet; introducing a liquid solution into the internalpathway so as to fill at least a portion of the internal pathway withthe liquid solution, the liquid solution comprising a solute capable ofundergoing thermal decomposition; heating the liquid solution while theliquid solution fills said at least a portion of the internal pathway toa sufficient temperature so that the solute undergoes thermaldecomposition to form a decomposition product within said at least aportion of the internal pathway; the heating forming a coatingcomprising the decomposition product on at least a part of the internalsurface wherein said at least a part of the internal surface borders theinternal pathway.

The use of a liquid solution rather than a sol may overcome orameliorate the problems associated with the viscosity and unstablenature of sols. Faster formation of the coating may also be possible.

In preferred embodiments, the current invention provides a coating thathas a relatively good homogeneity in terms of thickness.

In many instances, the current invention can be used to preparerelatively thick coatings (e.g. greater than 5 μm) in a single heatingstep. In addition, the current invention may allow formation of acoating extending along a relatively long internal pathway (e.g. greaterthan 1 m in length).

Preferably, the internal pathway is a channel which has first and secondopenings at the inlet and outlet respectively and which is fullyenclosed by the internal surface between the first and second openings.For example, the body may be a tube and the internal pathway may be aninternal channel or lumen of the tube. By way of another example, thebody is a cartridge.

In another preferred embodiment, the body has a plurality of channelsextending parallel to one another and between the inlet and the outlet.Each one of the channels has a first opening at the inlet and a secondopening at the outlet. Each channel may be fully enclosed by arespective internal surface between the first and second openings.Alternatively the parallel channels may be interconnected. The inlet maybe a formation which connects the first openings, such as a manifold orchamber. Alternatively, the inlet may simply be the first openings whenconsidered collectively. Likewise the outlet may be a formation whichconnects the second openings. Alternatively, the outlet may simply bethe second openings considered collectively.

Channels may be rectilinear but do not need to be.

In any embodiment which has one or more channels (such as a tube), theor each channel may have any practical cross-sectional shape. Forexample, the channel(s) may be circular in cross-section. In a bodywhich has a plurality of channels, the channels do not need to have thesame cross-sectional shape and/or size as one another. Thecross-sectional shape and/or size may vary along the length of achannel.

Each channel will have a maximum cross-sectional dimension. This issimply the largest dimension extending in a straight line across thechannel in a cross-sectional direction. Hence, when the channel has acircular cross-section, the maximum cross-sectional dimension will be adiameter. When the channel has a square cross-section, the maximumcross-sectional dimension will be a diagonal extending from one cornerof the square to the opposite corner. If the cross-section of thechannel is constant along the length of the channel, then the maximumcross-sectional dimension will be present all along the length of thechannel. Alternatively, if the channel cross-section varies along thelength of the channel, then the maximum cross-sectional dimension mayonly occur at a certain point or points along the length of the channel.Preferably, the or each channel has a maximum cross-sectional dimensionof less than 10 mm. In this case, the maximum cross-sectional dimensionmay be less than 5 mm, less than 4 mm, less than 3 mm, less than 2 mm,less than 1 mm, less than 0.5 mm, less than 0.4 mm, less than 0.3 mm,less than 0.2 mm or less than 0.1 mm.

A particularly preferred body is a tube which has a channel. The channelhas a cross-section which is circular and of constant diameter along thelength of the tube.

In an alternative preferred embodiment, the body is porous and theinternal pathway is formed by a plurality of interconnecting internalspaces within the body. In this case, the inlet may be a plurality ofexternal openings of the porous body. Alternatively, the inlet may be aformation, such as a chamber, which connects a plurality of externalopenings of the porous body. The outlet may be a plurality of externalopenings of the porous body (but different openings from those of theinlet). Alternatively, the outlet may be a formation, such as a chamber,which connects a plurality of external openings of the porous body.

Preferably, the internal pathway has a length of at least 0.1 m. In thiscase, the length may be at least 0.2 m, at least 0.5 m, at least 1 m, atleast 5 m, or at least 10 m.

In many cases, the internal pathway will be elongate having a lengthextending between the inlet and the outlet that is significantly longerthan the maximum cross-sectional dimension. One example of this is whenthe body is a tube. In such cases, a potential problem is that, duringthe heating stage, gas bubbles may form within the liquid solution andexpel liquid solution (i.e. in liquid form) from the internal pathway.The expulsion of liquid solution (in liquid form) is undesirable,although expulsion of gas formed from the liquid solution during heatingis generally inevitable and acceptable. For example, if the body is atube which is filled with a column of the liquid solution and the tubeis heated uniformly in an oven, gas bubbles may form in the column ofthe liquid solution causing expulsion of liquid solution. The followingpreferred embodiment avoids or minimises the expulsion of liquidsolution.

In this preferred embodiment, the internal pathway is elongate having alength extending between the inlet and the outlet. The portion of theinternal pathway that is filled with the liquid solution is filled withan elongate body of the liquid solution, the elongate body of liquidsolution having first and second ends. The heating is performed whileone of the inlet and the outlet is closed and the other one of the inletand the outlet is open. The first end of the elongate body of the liquidsolution lies closer than the second end to the open one of the inletand outlet along the length of the internal pathway, and the second endof the elongate body of the liquid solution lies closer than the firstend to the closed one of the inlet and outlet along the length of theinternal pathway. The heating comprises applying heat progressively tosuccessive regions of the elongate body of the liquid solution startingat the first end of the elongate body of the liquid solution and movingtowards the second end of the elongate body of the liquid solution. Inthis way, expulsion of liquid solution from the liquid pathway isreduced or prevented.

For example, when the internal pathway is the channel of a tube and theliquid solution fills the channel to form a column of liquid solution,the outlet of the tube may be blocked and the inlet is left open. Thecolumn of liquid solution has a first end near the inlet and a secondend near the outlet. The heating is then commenced at the first end ofthe column of the liquid solution. Only a small length of the column ofthe liquid solution undergoes significant heating at any one moment intime. The heated liquid solution forms a coating as discussed above andthis is generally accompanied by evaporation of a solvent of the liquidsolution. The gas formed during the evaporation escapes harmlesslythrough the open inlet of the tube. Because heating is commenced at thefirst end of the column of liquid solution, at or near the open inlet ofthe tube, the danger of expelling liquid solution in liquid form isavoided or reduced. This is because the formation of gas bubbles withinthe column of liquid solution and spaced from the end of the column isavoided or minimised because only solvent at the end of the column isevaporated. The application of heat progresses towards the closedoutlet. This is done sufficiently slowly so that only solvent at the endof the column of liquid solution evaporates. In this way, the formationof the coating and the generation of gas from evaporation of thesolvent, progress along the column of liquid solution.

The inlet or outlet may be closed in any convenient manner, such asbeing blocked in a reversible way by laboratory film, or by a valve.

The rate at which the application of heat moves progressively along theelongate body of liquid solution may preferably be from 10 μm to 10 cmper second, and more preferably from 0.1 mm to 10 mm per second, whilebeing sufficiently slow to prevent or reduce expulsion of the liquidsolution.

Preferably, when the body is a tube with the inlet at one end of thetube and the outlet at the other end of the tube, the localised heat isapplied by a heat source, and the tube and the heat source are movedrelative to one another to cause the progressive application of heat. Inthis case, the heat source may be annular and extend around thecircumference of the tube so that heat is applied simultaneously allaround the circumference of the tube.

This method of applying heat progressively is not essential. Instead,the expulsion of liquid solution may be avoided if the heating isperformed at a low temperature just sufficient to form the coating. Inthis case, the heat may be applied uniformly to the body, for example inan oven. Slower preparation times may result from this alternativemethod.

Preferably, in cases where only a portion of the internal pathway isfilled with the liquid solution, the portion that is filled forms atleast 1% of the length of the internal pathway, and preferably at least10%, or at least 20%, or at least 30%, or at least 40%, or at least 50%,or at least 60%, or at least 70%, or at least 80%, or at least 90% ofthe length of the internal pathway. Of course, the liquid solution mayfill the internal pathway completely.

According to a second aspect of the invention, there is provided amethod of forming a coating within a porous body, comprising: providinga porous body having an internal surface which defines a plurality ofinterconnected internal spaces within the porous body; introducing aliquid solution into the internal spaces so as to fill the internalspaces with the liquid solution, the liquid solution comprising a solutecapable of undergoing thermal decomposition; heating the liquid solutionwhile the liquid solution fills the internal spaces to a sufficienttemperature so that the solute undergoes thermal decomposition to form adecomposition product within the internal spaces; the heating forming acoating comprising the decomposition product on at least a part of theinternal surface.

The following preferred embodiments refer to any aspect of theinvention.

In one preferred embodiment, the decomposition product is a poroussolid. For example, the porous solid may be a porous metal oxide, or aporous carbon decomposition product.

The porous solid may be the only component of the coating. In this case,the body with the coating on the internal surface may be used, forexample, for chromatography. In particular, the method may be used toform a coating consisting of a porous solid and covering the internalsurface of a tube, such as a capillary tube, so as to form achromatography column.

Alternatively, the coating may comprise a porous solid decompositionproduct, such as a porous metal oxide, and one or more additionalcomponents. One preferred additional component is a catalyst, such ascatalytic metal particles. The porous solid decomposition product mayact as a support for the additional component(s). For example, anadditional component, such as a catalyst, may be entrapped within theporous solid decomposition product. Where the coating comprises a poroussolid decomposition product and an additional component, the additionalcomponent may itself be a decomposition product of a further solute inthe liquid solution.

Where it is desired to form a coating comprising a porous solid metaloxide decomposition product, the solute which decomposes to form themetal oxide preferably comprises a metal cation and an anion (or anotherligand). For example, the solute may be a dissolved metal salt. Themetal cation may be, for example, titanium, zinc, aluminium, magnesium,calcium, scandium, vanadium, chromium, manganese, iron, cobalt, nickel,copper, gallium, germanium, strontium, yttrium, niobium, molybdenum,cadmium, indium, tin, antimony, tellurium, barium, tantalum, tungsten,thallium, lead, bismuth, zirconium, and lanthanum or actinium groupmetals. The anion or the ligand may be, for example, nitrate, acetate,acetyl acetonate, nitrite, chloride, citrate, ammonia, carbonyl,cyclopentadienyl and its derivatives, and anions of organic acidsincluding amino acids. Salts consisting of one such cation and one suchanion decompose to form metal oxides at temperatures which areconvenient to achieve and which are sufficiently low to avoid damage tomany types of body to be coated.

Of the cations mentioned above, titanium, aluminium, magnesium,scandium, yttrium, zirconium, iron, cobalt, nickel, manganese, antimonyand tin tend to produce metal oxides which have a greater degree ofporosity. Zinc, calcium, vanadium, chromium, copper, gallium, germanium,strontium, niobium, molybdenum, cadmium, indium, tellurium, barium,tantalum, tungsten, thallium, lead, bismuth, and lanthanum or actiniumgroup metals tend to produce metal oxides which have a lower degree ofporosity.

Both higher and lower porosity metal oxides may be useful, although formany applications higher porosity metal oxides may be preferred.

For some applications metal oxides which have substantially no porositymay be desired.

Another type of porous solid decomposition product will be referred toas porous carbon decomposition product. Generally, a porous carbondecomposition product comprises at least 80% carbon which is not incompound form, when considered by weight as a percentage of the totalweight of the carbon decomposition product. Preferably, the porouscarbon decomposition product comprises an amount of carbon which is notin compound form, which is at least 90%, or at least 95%, or at least96%, or at least 97%, or at least 98% or at least 99%, by weight as apercentage of the total weight of the porous carbon decompositionproduct. The remainder, if any, of the porous carbon decompositionproduct may be, for example, carbon containing compounds or otherimpurities. Suitable solutes which decompose to form a porous carbondecomposition product are organic carbon containing compounds.Carbohydrates (and particularly sugars) are one example of organiccarbon containing compounds which undergo thermal decomposition to formporous carbon decomposition compounds. Particularly suitablecarbohydrates include glucose, fructose, galatose, ribose, maltose,sucrose, lactose. Derivatives of sugars may also be suitable. Onesuitable sugar derivative is sorbitol.

As discussed above, the coating may comprise a porous soliddecomposition product and a catalyst, the porous solid decompositionproduct being formed by thermal decomposition of the solute. In thiscase, to generate the catalyst component of the coating, the liquidsolution may also comprise a further solute comprising a further solutemetallic cation and a further solute anion (or a further solute ligand).The further solute metallic cation may be selected from the groupconsisting of: platinum, palladium, rhodium, osmium, iridium, ruthenium,copper, silver, cobalt, iron, nickel or gold. The further soluteundergoes thermal decomposition to form solid catalytic metal particles,during the same heating step used to convert the first-mentioned soluteto the porous solid decomposition product.

In another embodiment, the liquid solution comprises a first solute anda second solute. The second solute comprises a second solute metalliccation and a second solute anion (or a second solute ligand). The secondsolute metallic cation is selected from the group consisting of:platinum, palladium, rhodium, osmium, iridium, ruthenium, copper,silver, cobalt, iron, nickel or gold. During the heating step, the firstsolute undergoes thermal decomposition to form a porous solid, asdiscussed above. The second solute undergoes thermal decomposition to anoxide of the metal of the second solute metallic cation. Hence, theheating forms a coating comprising the porous solid derived from thefirst solute and a metal oxide derived from the second solute.Subsequently, the coating can be modified by converting the metal oxidederived from the second solute to solid catalytic metal particles byreduction of the metal oxide derived from the second solute with asuitable reducing agent.

In yet another embodiment, a coating comprising a porous solid is formedas described above, and the coating is then modified by introducingcatalytic metal particles into the pores of the porous solid in asubsequent step after the formation of the coating. For example, thesubsequent step may comprise introducing a further liquid solution intothe internal pathway, or into the internal spaces of the porous body. Inthis case, the further liquid solution comprises a solute capable ofbeing converted into catalytic metal particles. The conversion could be,for example, by way of thermal decomposition to metal particles, or byway of thermal composition to an oxide followed by reduction of theoxide to metal particles by a suitable reducing agent. Alternatively,the subsequent step may comprise introducing a sol containing the solidmetal catalytic particles into the internal pathway, or into theinternal spaces of the porous body, so that the solid metal catalyticparticles enter into and become entrapped within the pores of the poroussolid decomposition product.

In cases where the coating comprises a porous solid decompositionproduct, the liquid solution may comprise a molecule in solution whichacts to increase the mean pore diameter in the porous soliddecomposition product. The molecule may be a large molecule which may bedecomposed or oxidised during the heating step to leave a relativelylarge pore in the porous solid. Alternatively, the molecule may be alarge molecule which can be washed or evaporated away after theformation of the coating to leave a relatively large pore. The moleculemay be a polymer. One type of polymer which is particularly useful forincreasing the mean pore diameter is a block copolymer having thestructure poly(ethylene glycol)-poly(propylene glycol)-poly(ethyleneglycol). Block copolymers of this type are sold under the trade namePluronic (trade mark) by BASF. Pluronic F127 (trade mark) has been foundto be particularly useful for increasing the mean pore diameter. Otherpolymers that may be used to increase the mean pore diameter are: latex;poly(methyl methacrylate); polystyrene; and cross-linked polymers. Oneparticularly suitable cross-linked polymer ispolystyrene-divinylbenzene. Other large combustible molecules may beused.

Where a coating or a modified coating comprises catalytic metalparticles, the catalytic metal particles preferably contain at least 80%by weight of the metal not in the form of a compound, and morepreferably at least 90%, at least 95%, at least 96%, at least 97%, atleast 98% or at least 99% by weight of the metal not in compound form,as a percentage of the total weight of the metal particles.

The coating does not need to comprise a porous solid. For example, thecoating may be a metallic coating, which may have catalytic properties.In one such example, the solute comprises a metallic cation and an anion(or another ligand). The metallic cation is selected from the groupconsisting of: platinum, palladium, rhodium, osmium, iridium, ruthenium,copper, silver, cobalt, iron, nickel or gold. The decomposition productis metal in non-compound form which forms the metal coating.

For solutes which comprise a metallic cation selected from the groupconsisting of: platinum, palladium, rhodium, osmium, iridium, ruthenium,copper, silver, cobalt, iron, nickel or gold, suitable anions includeacetate, nitrate, nitrite, citrate, and chloride. Other suitable ligandsare carbonyl, ammonia and acetyl acetonate.

The concentration of the solute (or solutes) in the liquid solution canvary greatly. Generally, the greater the concentration, the thicker thecoating that is formed. Suitable concentrations may be in the range from0.1 wt/wt % to 80 wt/wt %.

As discussed above, the liquid solution has one or more solutesdissolved therein. Preferably, the liquid solution does not have anysuspended solid therein.

Preferably, in cases where only a part of the internal surface iscovered by the coating, the coating covers at least 50% of the internalsurface, and preferably at least 60%, or at least 70%, or at least 80%,or at least 90% of the internal surface. More preferably, the part ofthe internal surface that is covered by the coating is a continuouspart. Of course, the coating may cover the internal surface completely.

Preferably, the coating that is formed by the heating step has athickness of at least 1 μm, and more preferably at least 2 μm, or atleast 5 μm, or at least 10 μm, or at least 15 μm or at least 20 μm, inthe direction of the cross-section of the internal pathway. Given thatthe coating is formed in a single heating step, the coating will oftenbe homogenous throughout its thickness without having any discerniblelayers.

Preferably the liquid solution comprises a solvent selected from thegroup consisting of: water, methanol, ethanol, toluene, xylene,isopropanol, hexane, tetrahydrofuran, dimethylformamide, acetonitrile,dimethyl sulfoxide, and mixtures thereof. However, other suitablesolvents may be used. In many cases, solvents with relatively lowboiling points are preferred because they evaporate more quickly duringthe heating step and this may allow a shorter overall preparation time.The solvent should be chosen such that the solute or solutes of theliquid solution are sufficiently soluble in the solvent. In addition,the solvent should have a boiling point that allows evaporation at asufficient rate at a temperature that is sufficiently low to avoiddamage to the body. In addition, the solvent should preferably notdamage the coating or dissolve the coating to any significant extent.

Preferably, the body is formed from a material selected from the groupconsisting of: silica, steel, titanium, copper, aluminium, and plastics.However, other suitable materials may be used. The material should bestable at the temperature used for the heating step. The body may beformed form a plurality of materials.

As discussed above, during the heating step, the liquid solution isheated to a temperature sufficient to cause the thermal decomposition ofthe solute (or solutes). For example, the temperature may be equivalentto the thermal decomposition temperature of the solute plus a margin.The margin may be 50° C. to 100° C. Alternative, the margin may be, forexample, up to 500° C. The temperature should also be sufficient tocause evaporation of the solvent of the liquid solution at an acceptablerate, but not too high to damage the material, or materials, of thebody.

The method may be used in many applications. For example, the method maybe used to provide a tube or cartridge or other body with a coating soas to form a chromatography column. In this case, it may be sufficientfor the coating to consist of a porous solid without any additionalcomponents. Alternatively, the method may be used to coat an internalsurface with a catalytic coating so that chemical reactions catalysed bythe coating may be performed in an internal pathway, or within theinternal spaces of a porous body. This may be particularly useful forcoating internal channels of micro-reactors or mili-reactors.

The following is a more detailed description of embodiments of theinvention, by way of example, with reference to the appended drawings,in which:

FIG. 1 is a schematic representation of a heating apparatus for applyingheat progressively along a capillary tube;

FIG. 2 is a cross-sectional scanning electron microscope image showing aporous coating on an internal surface of a capillary tube;

FIG. 3 is a graphical representation of a coating thickness distributionalong the axis of the capillary tube shown in FIG. 2; and

FIG. 4 is a graphical representation of a pore diameter distribution ofthe porous coating shown in FIG. 2.

Referring to FIG. 1, the heating apparatus comprises an annular heatingelement 10, a stepper motor 11, and a pair or rollers 12, 13. Therollers 12, 13 are mounted on the stepper motor 11 and spaced from oneanother so that a capillary tube 14 can be placed between the rollers12, 13 and gripped by the rollers 12, 13. The rollers 12, 13 are drivenby the stepper motor 11 so that the rollers 12, 13 can displace thecapillary tube 14, when so gripped, in an upward direction as shown inFIG. 1.

The annular heating element 10 has an internal opening 15 having adiameter large enough to receive the capillary tube 14 so that theheating element 10 lies close to the external surface of the capillarytube 14. The rollers 12, 13 and the heating element 10 are positioned sothat the capillary tube 14 is received in the opening 15 of the heatingelement 10 while the capillary tube is gripped by the rollers 12, 13. Asshown in FIG. 1, the heating element 10 extends around the externalcircumference of the capillary tube 14. The heating element 10 iscapable of applying heat to the capillary tube 14 all around thecircumference of the capillary tube 14.

EXAMPLE 1

The following is an example of the use of the method to form a coatingon the internal surface of a capillary tube. The heating apparatus shownin FIG. 1 is used to perform the heating step of the method.

A fused silica capillary tube (10 m long, 0.53 mm i.d.) was filled witha liquid solution of zinc (II) nitrate hexahydrate (6.0 g,Sigma-Aldrich, 98%), palladium (II) acetate (0.130 g, Sigma-Aldrich,98%), Pluronic F127 (0.4 g, Sigma-Aldrich) in methanol (15 mL, FischerScientific, 99.9%).

The capillary tube 14 was then closed at one end with a shut off valve(not shown) and the other end was left open. The capillary tube 14 wasplaced between the rollers 12, 13 of the heating apparatus shown in FIG.1 with the open end of the capillary tube 14 located upwards and withthe open end lying within the internal opening 15 of the annular heatingelement 10. The temperature of the annular heating element was set to300° C.

The capillary tube 14 was then moved upwardly through the internalopening 15 of the heating element 10 at a constant displacement speed of3 mm s⁻¹. During this process, the volume of the liquid solution locatedwithin the annular heating element 10 at any point in time was heatedand formed a coating on the internal surface of the capillary tube 14.The methanol solvent within the heated volume evaporated and wasexpelled as a gas from the open end of the tube. As the capillary tubewas displaced upwardly, the application of heat moved along thecapillary tube 14 progressively towards the lower closed end of thecapillary tube 14 so that the formation of the coating and theevaporation of the methanol solvent also progressed towards the lowerclosed end of the capillary tube 14.

The capillary tube 14 was then washed with methanol (100 μL min⁻¹ for 60min) and dried at 110 ° C.

After the washing step, the capillary tube 14 had a continuous coatingwhich covered the entire internal surface of the capillary tube 14. Themass of the coating obtained was 6.5 mg m⁻¹ and the palladium loadingwas 3.4% by mass as a percentage of the total mass of the coating. Thecoating consisted of zinc oxide formed by thermal composition from thezinc nitrate together with palladium particles formed by thermaldecomposition from the palladium acetate.

FIG. 2 shows a representative scanning electron microscopy (SEM)cross-sectional image of the capillary tube 14 with the coatingconsisting of plate-like crystallites of ZnO. As determined by X-raydiffraction (results not shown), the coating comprised hexagonal zincitecrystallites 25±4 nm in diameter. Transmission electron microscopyshowed the palladium nanoparticles to be 3-5 nm in diameter

Studies on the axial distribution of the coating thickness (see FIG. 3)showed that the coating was uniform with an average thickness of 3.6 μmand standard deviation of 2.4 μm. The maximum coating thickness observedwas less than 10 μm, so no narrow places in the capillary tube 14 werepresent where high pressure drop and pore diffusion limitations mightotherwise have been expected. These data agree with very low pressuredrop (of 0.2 bar) in the system when the coated capillary tube was usedfor a hydrogenation reaction as described below in Example 2.

Nitrogen physisorption studies (see FIG. 4) showed that the coating wasmesoporous with an average BJH desorption pore diameter of 26 nm and BETspecific surface area of 16 m²g⁻¹.

EXAMPLE 2

Solvent-free hydrogenation of 2-methyl-3-butyn-2-ol (MBY) was performedusing the coated capillary tube 14 prepared in Example 1. Briefly,hydrogen and MBY from a mass-flow controller and a syringe pump,respectively, were combined in a T-joint and passed through the coatedcapillary tube 14 which was placed in a temperature-controlled waterbath. The flow rate of hydrogen was constant, 19 mL min⁻¹ (STP), whilethe reaction temperature and the MBY flow rate were varied to optimisethe 2-methyl-3-buten-2-ol (MBE) yield.

For every reaction temperature, MBE yield achieved a maximum at acertain MBY flow rate, where the product of MBY conversion and the MBEselectivity was the highest. At a lower MBY flow rate, the residencetime increased leading to over-hydrogenation to 2-methyl-2-butanol(MBA); while at a higher MBY flow rate the decreased residence time leadto lower MBY conversion. The hydrogenation rate increased withtemperature and resulted in the shift of the maximum MBE yield to higherflow rates. Regardless of the reaction temperature, the maximum MBEyield was above 95% and MBE selectivity was higher than 98% up to 90%MBY conversion. High selectivity of Pd/ZnO catalysts was caused by thein situ formation of an PdZn alloy resulting in the decreased adsorptionof alkene species on the catalyst surface.

EXAMPLE 3

The following is an example of the use of the method to form a coatingwithin a porous body.

First a porous body was prepared as follows. A stainless steel tubehaving an external diameter of 6.35 mm and an internal diameter of 4.2mm was filled with porous cylinders formed from silicon carbide (SiC)foam. Each cylinder had an external diameter of slightly less that theinternal diameter of the tube, so as to be a close fit within the tube,and a height of about 5 mm. In total, enough silicon carbide foamcylinders were used, end-to-end within the tube, to fill a 20 cm lengthof the stainless steel tube. In this way, a porous body comprising thestainless steel tube and the silicon carbide foam cylinders is formed.Each silicon carbide foam cylinder has an internal surface which forms aplurality of interconnecting internal spaces, and the interconnectinginternal spaces form an internal pathway through the foam cylinder fromone end the foam cylinder to the other end.

The porous body was then flushed by passing petroleum ether and acetonedown the filled tube while subjecting the porous body to ultrasonictreatment. This flushing step serves to remove surface contaminants.

The tube was then filled with an aqueous solution of 5 wt% zinc nitratehexahydrate and closed at one end with a valve. The aqueous solutionfilled the internal spaces of the silicon carbide foam cylinders.

The tube, filled with the zinc nitrate solution, was then displaced,open end first, at a rate of 0.1 mm/s, into a vertical tube furnace heldat a temperature of 350° C. Once the tube had been introduced completelyin the furnace it was left within the furnace for a further 30 minutes.The application of heat caused the zinc nitrate to undergo thermaldecomposition to form a zinc oxide coating. The water was evaporated andexpelled harmlessly from the open end of the tube as gas.

The porous body was then flushed with water to remove loosely boundmaterial and dried at 120° C. in an oven for 2 hours.

After drying, the internal spaces of the silicon carbide foam cylinderswere coated with zinc oxide. The mass of the coating obtained was 70 mgfor the 20 cm length filled tube.

1. A method of forming a coating within an internal pathway, comprising:providing a body having an internal surface which defines an internalpathway within the body, the body having an inlet and an outlet bothcommunicating with the internal pathway for passage of a fluidsuccessively into the inlet then through the internal pathway and thenout of the outlet; introducing a liquid solution into the internalpathway so as to fill at least a portion of the internal pathway withthe liquid solution, the liquid solution comprising a solute capable ofundergoing thermal decomposition, wherein said at least a portion of theinternal pathway forms at least 50% of the length of the internalpathway; heating the liquid solution while the liquid solution fillssaid at least a portion of the internal pathway to a sufficienttemperature so that the solute undergoes thermal decomposition to form adecomposition product within said at least a portion of the internalpathway; the heating forming a coating comprising the decompositionproduct on at least a part of the internal surface wherein said at leasta part of the internal surface borders the internal pathway.
 2. Themethod according to claim 1, wherein the internal pathway is a channelwhich has first and second openings at the inlet and outlet respectivelyand which is fully enclosed by the internal surface between the firstand second openings. 3-8. (canceled)
 9. The method according to claim 1,wherein the internal pathway is elongate having a length extendingbetween the inlet and the outlet, wherein said at least a portion of theinternal pathway that is filled with the liquid solution is filled withan elongate body of the liquid solution, the elongate body of liquidsolution having first and second ends, wherein said heating is performedwhile one of the inlet and the outlet is closed and the other one of theinlet and the outlet is open, the first end of the elongate body of theliquid solution lying closer than the second end to the open one of theinlet and outlet along the length of the internal pathway and the secondend of the elongate body of the liquid solution lying closer than thefirst end to the closed one of the inlet and outlet along the length ofthe internal pathway, wherein said heating comprises applying heatprogressively to successive regions of the elongate body of the liquidsolution starting at the first end of the elongate body of the liquidsolution and moving towards the second end of the elongate body of theliquid solution, whereby to reduce or prevent expulsion of liquidsolution from the liquid pathway.
 10. The method according to claim 9,wherein the body is a tube with the inlet at one end of the tube and theoutlet at the other end of the tube, wherein said heat is applied by aheat source, and wherein the tube and the heat source are moved relativeto one another to cause said progressive application of heat.
 11. Themethod according to claim 10, wherein the heat source is annular andextends around the circumference of the tube and said heat is appliedsimultaneously all around the circumference of the tube. 12-14.(canceled)
 15. A method of forming a coating within a porous body,comprising: providing a porous body having an internal surface whichdefines a plurality of interconnected internal spaces within the porousbody; introducing a liquid solution into the internal spaces so as tofill the internal spaces with the liquid solution, the liquid solutioncomprising a solute capable of undergoing thermal decomposition; heatingthe liquid solution while the liquid solution fills the internal spacesto a sufficient temperature so that the solute undergoes thermaldecomposition to form a decomposition product within the internalspaces; the heating forming a coating comprising the decompositionproduct on at least a part of the internal surface.
 16. The methodaccording to any preceding claim 1, wherein the decomposition product isa porous solid. 17-20. (canceled)
 21. The method according to claim 16,wherein the coating comprises the porous solid decomposition product anda catalyst supported by the porous solid decomposition product, whereinthe catalyst comprises metal particles, the metal particles beingentrapped within the porous solid decomposition product.
 22. The methodaccording to claim 1 wherein the solute comprises a metallic cation, andwherein the decomposition product is a metal oxide.
 23. The methodaccording to claim 22, wherein the metallic cation is selected from thegroup consisting of: titanium, zinc, aluminium, magnesium, calcium,scandium, vanadium, chromium, manganese, iron, cobalt, nickel, copper,gallium, germanium, strontium, yttrium, niobium, molybdenum, ruthenium,rhodium, palladium, silver, cadmium, indium, tin, antimony, tellurium,barium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold,mercury, thallium, lead, bismuth, zirconium, and lanthanum or actiniumgroup metals.
 24. (canceled)
 25. The method according to claim 22,wherein the solute comprises an anion or a ligand, and wherein the anionor the ligand is selected from the group consisting of: nitrate,acetate, acetyl acetonate, nitrite, chloride, citrate, ammonia,carbonyl, cyclopentadienyl and its derivatives, and anions of organicacids including amino acids. 26-29. (canceled)
 30. The method accordingto claim 22, wherein the liquid solution comprises a further solutecomprising a further solute metallic cation, the further solute metalliccation being selected from the group consisting of: platinum, palladium,rhodium, osmium, iridium, ruthenium, copper, silver, cobalt, iron,nickel or gold, and wherein said method comprises converting the furthersolute to form metal particles.
 31. (canceled)
 32. The method accordingto claim 30, wherein after said conversion the metal particles areentrapped within the decomposition product of the said solute. 33-34.(canceled)
 35. The method according to claim 1, wherein the solutecomprises a metallic cation, wherein the metallic cation is selectedfrom the group consisting of: platinum, palladium, rhodium, osmium,iridium, ruthenium, silver, rhenium, mercury or gold, and wherein saiddecomposition product is metal in non-compound form. 36-37. (canceled)38. The method according to claims 16, wherein the coating consistssubstantially of said porous solid decomposition product, and whereinthe method comprises the further step, performed subsequently to theformation of said coating, of providing metal particles within pores ofsaid porous solid decomposition product. 39-41. (canceled)
 42. Themethod according to claim 15, wherein the decomposition product is aporous solid.
 43. The method according to claim 42, wherein the coatingcomprises the porous solid decomposition product and a catalystsupported by the porous solid decomposition product, wherein thecatalyst comprises metal particles, the metal particles being entrappedwithin the porous solid decomposition product.
 44. The method accordingto claim 15, wherein the solute comprises a metallic cation, and whereinthe decomposition product is a metal oxide.
 45. The method according toclaim 44, wherein the liquid solution comprises a further solutecomprising a further solute metallic cation, the further solute metalliccation being selected from the group consisting of: platinum, palladium,rhodium, osmium, iridium, ruthenium, copper, silver, cobalt, iron,nickel or gold, and wherein said method comprises converting the furthersolute to form metal particles.
 46. The method according to claim 45,wherein after said conversion the metal particles are entrapped withinthe decomposition product of the said solute.